1. Field of the Invention
The present invention relates to a sheet stacking apparatus and an image forming apparatus, and more particularly to such an apparatus adapted to discharge and stack selectively sheets on plural sheet stacking portions, thereby providing a sheet stacking apparatus and an image forming apparatus capable of sheet stacking without destructing a bundle of stacked sheets and without causing a stain or a damage by friction in the sheets.
2. Description of the Related Art
In an image forming apparatus for forming an image on a sheet, a higher speed in image formation is recently intended as a result of advances in the technology. As a result of such higher speed in image formation, the sheet discharged from a main body of the image forming apparatus is discharged at a higher speed, so that, in a sheet stacking apparatus of a large capacity for stacking the discharged sheets, requested are not only a large capacity but also a highly precise sheet stacking.
Among the prior image forming apparatuses, there is known one enabling a stacking of large capacity, by arranging a plurality of such large-capacity sheet stacking apparatuses in parallel (cf. Japanese Patent Application Laid-open No. 06-144682). In the case that such large-capacity sheet stacking apparatuses are arranged in parallel, when a sheet is to be stacked in a downstream sheet stacking apparatus, the sheet is passed through the upstream sheet stacking apparatus and is guided to the downstream sheet stacking apparatus.
In such case of stacking a sheet in the downstream sheet stacking apparatus, the conveyed sheet may cause a hanging-down or a flapping at the trailing end thereof, thereby liable to cause a destruction of a sheet bundle already stacked in the upstream sheet stacking apparatus or a stain or a damage to the sheet by friction. For this reason, there is known a sheet stacking apparatus, at the upstream side, having a sheet guide above the already stacked sheet bundle in order to avoid the destruction of the already stacked sheet bundle or the stain or the damage to the sheet by friction.
On the other hand, the sheet stacking apparatus is recently requested to stack a large amount of sheets without increasing the dimension of the apparatus. For this reason, an increase in the capacity is intended for example by providing a sheet stacking apparatus with plural sheet stacking portions and, in the case of discharge of sheets of a small size such as A4-size, stacking such sheets in the respective sheet stacking portions. Also in the case of stack of a large-sized sheet such as A3-size, the stacking of such large-sized sheets is made possible by stacking such sheets bridging plural sheet stacking portions.
In the case that such plural sheet stacking portions are disposed within a single sheet stacking apparatus, the sheets are discharged and stacked selectively on such plural sheet stacking portions. When a sheet stacking portion at the downstream side is selected for sheet discharge, the sheet is made to pass through the upstream sheet stacking portion and directed to the sheet stacking portion at the downstream side.
However, in the case of sheet stacking on the downstream sheet stacking portion, the conveyed sheet may have a hanging-down or a flapping of the trailing end thereof thereby causing the sheets, already stacked in the upstream sheet stacking portion, to be moved together. Such movement together of the sheets may cause a destruction of the sheet bundle or a stain or a damage to the sheet by the friction.